Carboxylic anhydrides are produced by gas phase oxidation of aromatic hydrocarbon materials, with a typical example being the production of phthalic anhydride from o-xylene or naphthalene.
For use in the production of phthalic anhydride from o-xylene by means of fixed bed gas phase oxidation, various catalysts have been reported (for example, Practical catalysts for Respective Reactions, p. 358, 1970 , edited by Kimio Tarama, published by Kagaku Kogyo-sha, Japan) including a combination of vanadium pentoxide with titanium oxide (anatase type), a combination of vanadium pentoxide with other active metal oxide such as tellurium oxide, molybdenum oxide, tungsten oxide, nickel oxide, niobium oxide, tin oxide, chromium oxide or the like, and a combination of vanadium pentoxide with an alkai metal salt such as of potassium, lithium, sodium or the like, supported on inert carriers such as Alundum, silicon carbide, quartz, pumice, .alpha.-alumina and the like.
In the case of a gas phase oxidation process in which a fixed catalyst bed is employed, excessively high heat generated by exothermic oxidation reaction is removed by employing a system in which a catalyst is uniformly packed into several thousands of reactor pipes, each having a small diameter of about 1 inch, and the exterior of the pipes is filled with a heat transfer medium for cooling use. This system, however, requires considerable labor and cost to complete uniform packing of a catalyst into each of such a massive number of reactor pipes, as well as great burdens of the cost of equipment and the operation management to maintain pressure loss and temperature of each of these reactor pipes at constant levels. This system also requires considerable labor and cost when deteriorated catalyst is replaced by fresh catalyst.
Also, a catalyst in which active components are coated on an inert carrier is apt to cause a run-away reaction triggered by ununiformity of reaction due to channeling of reaction gas, formation of hot spots, increased pressure loss and the like caused by peeling and release of the active components from the carrier during packing of the catalyst or at the time of the operation. In addition, high productivity cannot be attained by the use of the fixed bed process because concentration of a reaction gas must be maintained within its explosion limit and, therefore, the reaction gas can be supplied only at a low concentration level.
For the purpose of solving such problems, it is preferable to perform gas phase oxidation using a fluidized catalyst bed.
Compared to the aforementioned fixed bed process, such a fluidized bed process is markedly advantageous, because not only generated heat by the exothermic oxidation reaction can be removed easily but also channeling of the flow of material gas and formation of hot spots both of which are common in the case of the fixed bed process can be avoided. Another advantage of the fluidized bed process is that exchange and supplement of a catalyst can be made with less labor and cost. The fluid bed process still has a great advantage from a view point of productivity because of a possibility to increase concentration of a reaction material.
For use in the production of phthalic anhydride from o-xylene by means of gas phase oxidation, as well as the case of using naphthalene as the starting material, various fluid catalysts have been reported, for example in B.P. 941,293 (1963) and U.S. Pat. No. 3,232,955 (1966), such as a combination of vanadium pentoxide with potassium sulfate and other combinations with molybdenum oxide, tungsten oxide, phosphorus oxide, boron oxide and the like supported on silica. The use of such silica-supported catalyst, however, causes excess oxidation reaction and side reactions which results in the formation of CO and CO.sub.2, thus making it difficult to obtain phthalic anhydride in a high yield. In order to improve the phthalic anhydride yield, attempts to mix the reaction gas with a halogen gas such as Br.sub.2 have been reported for example in D.P. 1,144,709 (1963) and U.S. Pat. No. 3,455,962 (1969), but the use of such a halogen gas causes corrosion of equipment and therefore results in operational troubles.
A number of catalysts in which titanium oxide is used as a carrier and vanadium pentoxide is supported on the carrier have been proposed for example in B.P. 1,067,726 (1967) and Fr.P. 1,537,351 (1968). A catalyst having certain mechanical strength can be obtained by making a fused body of titanium oxide and vanadium pentoxide together with ammonium thiocyanate or an alkali compound. However, specific surface area and pore volume of the catalyst decrease by the formation of fused body, thus resulting in significant reduction of the catalytic activity. Because of the reduced activity, such a fused catalyst requires a high reaction temperature which causes excess oxidation and side reactions. In consequence, it is difficult to obtain phthalic anhydride with a high yield by the use of the fused catalyst.
In addition, since the formation of a fused body results in a catalyst having markedly high bulk density amplified by the high specific gravity of titanium oxide, it is difficult to perform efficient fluidized bed reaction using such a high bulk density catalyst.
Because of these reasons, production of phthalic anhydride from o-xylene by means of a fluidized bed gas phase oxidation reaction has not been put into practical use.